This invention relates to a method of operating a primary fractionator, also termed a quench tower, in an ethylene plant with anti-foulants, particularly defoamers.
In the manufacture of lighter hydrocarbon products such as ethylene, heavier hydrocarbons such as naphtha or diesel oil are cracked in pyrolysis heaters at temperatures of approximately 850° C. to form mixtures of smaller molecules including, but not limited to, ethylene, propylene, and butadiene. Such mixtures, commonly termed cracked gases, are cooled in various stages of an ethylene plant until they are separated in the fractionation section of the ethylene plant
During primary heat recovery, the cracked gases pass through and are cooled by a series of heat exchangers, also termed transfer line exchangers, before being quenched with a heavy oil. This heavy oil, which is known as quench oil or bottoms quench oil, accumulates in the bottom of the primary fractionator. The primary fractionator contains varying components of fuel oil species ranging from the bottoms of the column to the beginning of what is called the rectification section of the column. The rectification section of the column is prone to severe fouling problems related to species such as styrene, indene, di-vinyl benzene, alpha-methyl styrene, indene derivatives, naphthalene and other higher ring compound derivatives.
The polymerization products of these species deposit at a very rapid rate not only on upper tray surfaces, but also beneath tray surfaces. Due to this fouling problem, an increase in column pressure drop along with reduction in fractionation efficiencies are experienced. Consequently, the quality of gasoline condensing in the quench water tower and also the quality of fuel oil made from the system are negatively effected. Typically, the problem of fouling in the rectification section is also accompanied by poor viscosity control in the bottom of the quench oil tower due to improper operations of the primary fractionator. The deposition of the fouling species, commonly as polymers, obstructs the vapor and liquid flow inside the fractionator and due to the reduced surface area available, the environment is conducive to increased froth/foam generation in the column. The presence of foam further increases the pressure drop and results in entrainment of higher boiling point products into the lighter products and vice versa. Due to excessive foaming, the column pressure drop increases very rapidly and is a major reason for reduced unit efficiencies.
As fouling continues to occur, the rate of foaming also increases in the column resulting in plant operators having to reduce unit feed rates significantly and, ultimately, shut down the plant for cleaning the primary fractionator.
The present invention provides a method to operate primary fractionators which are fouled due to polymer deposition by providing an appropriate chemical agent to extend run-length of the primary fractionator.